Alright, I just meant "Not now"...would Ní anois be grammatical there as well?

Yes. You can leave anything out following a focussed phrase as ní anois, Ní anois (atá mé á dhéanamh).So, ní anois on its own is grammatical. But you must be aware: It's an ellipsis.

English "not now" isn't an ellipsis (or rather a different kind of). In English, "not" is an adverb. It can negate any sort of words (not I, not him, not that, ...)

But Irish "ní" is a verbal particle or a copula form. As a verbal particle, it can only negate finite verbs (ní dhéanann sé, ni thuigim, ní fhuil tú > níl tú)As a copula form, it can only negate copula sentences.

One final question: are there much more indirect ways of saying it (perhaps sula dtarlaíonn sé nach...), and if not, is ar eagla nach the best translation?

sarar dtarlaíonn sé = lest it happens.I had a look in Dinneen Dictionary and sarar is given as an alternative for sular which it gives for lest. you may download a free version of Dinneen' s dictionary and it is excellent .McKenna English-Irish Dictionary also gives SAR for lest